We have used 3T3-L1 fibroblasts for studying hormonal regulation of lipoprotein lipase (LPL) in adipocytes. Earlier studies showed that insulin is most important in expression of the fat cell phenotype, development of LPL and deposition of triacylglycerol (TG). Insulin is also the primary regulatory factor of LPL in mature 3T3-L1 fat cells: It stimulates at once LPL activity. We recently found that antibodies to human insulin receptors duplicate effects of insulin on the enzyme in 3T3-L1 cells. 3T3-L1 fat cells also contain hormone-sensitive lipase, which hydrolyzes intracellular TG. Epinephrine and norepinephrine both stimulated release of glycerol from cells without affecting LPL activity. Although insulin treatment for 18 h stimulated LPL activity in these cells, it did not affect stimulation of glycerol release by catecholamines. The mechanisms for controlling these LPL and hormone-sensitive lipase appear to be separate and distinct. We are now producing antibodies to purified LPL of 3T3-L1 fat cells. The antibodies neutralize activity of the enzyme and will be used to study induction of LPL synthesis in 3T3-L1 fat cells.